The present invention relates to an apparatus for controlling electric cars which are provided with d-c shunt motors as drive motors, and particularly to a control apparatus employing choppers.
Conventional apparatus for controlling electric cars using choppers generally include systems employing a field chopper and d-c compound motors, and systems employing a chopper and d-c series motors. With the former systems employing compound motors, however, the motors are cumulatively connected or differentially connected to exhibit different characteristics with series field and shunt field in the powering condition and regenerative condition. Further, since the armature circuit is controlled by resistors, a cam-type controller must still be employed. With the latter systems on the other hand, a switching device is required for switching the powering and braking, and for switching the forward running and backward running. Further, it is not allowed to continuously control the field, and the motor voltage must be maintained to be smaller than a voltage of stringing (power source) during the regenerative breaking condition. Therefore, it is not possible to stably control the braking when running at high speeds.
In order to solve such problems, there has been disclosed and proposed in the previously field U.S. patent application Ser. No. 451,651 a new apparatus for controlling electric cars employing d-c shunt motors as drive motors.
According to the above-proposed apparatus for controlling electric cars, the field circuit controls both the forward rotation and the reverse rotation, the armature circuit controls a chopper which is provided in series with the armature to control the armature current at the time of acceleration in a region where the armature voltage is lower than the stringing (power source) voltage, and selects and controls the chopper in the field circuit in a region where the armature voltage is nearly equal to, or greater than, the stringing voltage. Further, the chopper in the armature circuit works to balance the unbalanced current among the circuits of electric motors. Thus, it is made possible to employ shunt motors for driving the electric cars. Accordingly, it is possible to switch the forward running and backward running, as well as to switch the powering and braking, by the field circuit. Further, even when the armature voltage becomes equal to or higher than the stringing (power source) voltage during the regenerative braking condition, the regenerative brake can be effectively applied by the field control.
Even the above-mentioned control apparatus, however, presents a problem as described below. That is, with the apparatus for controlling electric cars, the system for controlling armature currents and field currents must be within the main circuit limits and must remain stable. The above control apparatus, however, has shunt characteristics in which the armature current drops with respect to the axis of speed under the condition where a constant current flows through the chopper of the armature. Therefore, it is virtually impossible to select the constants of the system. It can therefore be contrived to relax the shunt characteristics by inserting a resistance in series with the armature. According to this method, however, heat loss increases due to resistance.